1. Field of the Invention
The present invention relates to polarizable electrodes for an electric double layer capacitor and to methods for production thereof, and relates to methods for producing the electric double layer capacitor.
2. Description of Related Art
An electric double layer capacitor is used as a backup power source, a car battery, etc., attributed to its large farad level capacity and excellent charging and discharging characteristics.
An electric double layer capacitor includes a pair of polarizable electrodes, a separator disposed between the pair of polarizable electrodes, and an electrolyte. Each of the members of the pair of polarizable electrodes, which are separated by the separator, functions as an anode or a cathode. As a material for the polarizable electrodes of the electric double layer capacitor, activated carbon having fine pores is generally used. An electric double layer is formed when ions contained in the electrolyte are absorbed in the fine pores of the activated carbon so as to form the anode and cathode of the capacitor.
As a method for producing the above-mentioned polarizable electrodes, Japanese Patent Application, Second Publication, No. Hei 7-105316 discloses a method which includes a step of mixing an activated carbon powder, a fluorine-containing polymer resin, and a liquid lubricant (a solvent for lubrication) to form a sheet of the mixture, and a step of subsequently removing the liquid solvent by heating, extraction, and so forth.
Since an electric double layer capacitor which includes the activated carbon as an electrode material and an organic electrolyte, is generally used when it is charged to the level of about 2.5V, there is a danger that the internal resistance of the capacitor will increase due to a reaction between water and the electrolyte if water remains in the capacitor. Accordingly, when an electric double layer capacitor is produced, it is necessary to decrease the amount of water in the capacitor to be as low as possible.
Since the activated carbon has an excellent property of absorbing chemical substances, it is assumed that water components in the capacitor are present in a state being absorbed by the activated carbon. Accordingly, the polarizable electrodes may be sufficiently dried in order to decrease the amount of water in the capacitor. However, there is a danger that the strength of the electrodes may be reduced, if a drying process is carried out excessively, due to thermal denaturing of a binder resin contained in the polarizable electrodes.
Also, there in no disclosure in the above Japanese Patent Application, Second Publication, No. Hei 7-105316 about practical manners and degree of decreasing the amount of water.